1. Field of the Invention
The present invention relates to pH-sensitive polymers which are changeable in physical properties, such as swellability and solubility, depending on pH and a method for preparing the polymers.
2. Description of the Prior Art
Generally, pH-sensitive polymers are water-soluble with ionizable functional groups. Their physical properties, such as solubility, swellability, etc., are changed depending on pH. Since the report for the phase transition of pH-sensitive polymers in Nature, 165, 414 (1950), many pH-sensitive polymers have been developed (see, Journal of Controlled Release, 15, 141 (1991)), most of which contain functional groups sensitive to pH, for example, carboxylic groups of weak acidity or tertiary amino groups of weak basicity.
Monomers of the pH-sensitive polymers developed thus far, include acrylic acid, methacrylic acid, sodium styrene sulfonate, sulfoxyethyl methacrylate, aminoethyl methacrylate, N,N-diethylaminoethyl methacrylate, N,N-dimethyl aminoethyl methacrylate, vinylpyridine, vinylbenzyl trimethylammonium chloride, etc. That is, pH-sensitive polymers may be prepared by polymerizing any one or combinations of these monomers to homopolymers or copolymers. These pH-sensitive polymers may have a structure of linear polymer, grafted copolymer, hydrogel or interpenetrating network polymer.
Carboxylic acid-containing polymers, which are most extensively developed, show pH sensitivity in a pH range of 4-6 owing to the intrinsic pKa values of the carboxylic acid. They, however, are limited in pharmaceutical application because the pKa values do not reach the physiological pH values of the human body.